Drug eluting stents have become popular medical devices. One difficulty with such devices is the difficulty in binding the drug to the stent and controlling drug elution. Previously, this problem has been addressed by incorporating the drug, often rapamycin, sirolimus, or the like, within a carrier and layering the drug carrier on a stent surface.
The carrier coating process generally requires a solvent, in which the drugs are usually soluble, and it is therefore desirable to provide a drug coated stent without polymeric vehicles to deliver the drug.
The biocompatibility of polymers has come into question. A degradable coating may provide biocompatibility advantages over permanent polymer coatings in allowing the tissue to eventually come into direct contact with the bulk stent material.
Additionally, carriers and therapeutic agents dry at different rates or are carried with the solvent as the solvent diffuses through the carrier. This results in formation of a concentration gradient through the thickness of the coating, as the carrier dries first, forcing the still wet therapeutic agent to the outer edges of the coating. While the coating still contains the same amount of the drug, the resulting concentration of the therapeutic agent near the outer surface of the coated stent can result in a “drug burst” on deployment. This drug burst must be considered when establishing dosage and elution characteristics.
It would be desirable, therefore, to provide a coated stent that would overcome the limitations and disadvantages inherent in the devices described above.